1. Field of the Invention
The present invention relates to methods of producing heterologous biological substances in enzyme-deficient Aspergillus niger mutant strains, methods of obtaining the enzyme-deficient Aspergillus niger mutant strains, and the enzyme-deficient Aspergillus niger mutant strains.
2. Description of the Related Art
Aspergillus niger secretes large quantities of glucoamylase. However, Aspergillus niger hosts with the desirable traits of increased protein expression and secretion may not necessarily have the most desirable characteristics for successful fermentation. The fermentation may not be optimal because of the secretion of multiple enzymes requiring removal during the recovery and purification of a biological substance of interest or the enzymes may co-purify with the biological substance.
Boel et al., 1984, EMBO J. 3: 1097-1102, 1581-1585, disclose the cloning of the glucoamylase (glaA) gene of Aspergillus niger. Fowler et al., 1990, Curr. Genet. 18: 537-545 disclose the deletion of the glucoamylase (glaA) gene of Aspergillus niger. 
Korman et al., 1990, Curr. Genet. 17: 203-217 disclose the cloning, characterization, and expression of two alpha-amylase genes (amyA and amyB) from Aspergillus niger var. awamori. U.S. Pat. No. 5,252,726 discloses the cloning of two full length neutral alpha-amylase genes from Aspergillus niger. 
U.S. Pat. No. 5,252,726 discloses the cloning of a portion of an acid stable alpha-amylase gene (asa) from Aspergillus niger. 
Pedersen et al., 2000, Metabolic Engineering 2: 34-41, and WO 00/50576 disclose the disruption of an oxatoacetate hydrolase (oah) gene encoding oxaloacetate hydrolase (EC 3.7.1.1) in a glucoamylase-producing strain of Aspergillus niger, wherein the resulting strain was incapable of producing oxalic acid.
WO 01/68864 discloses that prtT-disrupted Aspergillus niger strains are protease deficient, indicating that deletion of prtT expression in a host strain can result in an increase in the level of recoverable protein susceptible to proteolysis.
It is an object of the present invention to provide improved Aspergillus niger hosts which combine the capacity for expression of commercial quantities of a biological substance while being deficient in the production of enzymes which can complicate recovery and downstream processing of the biological substance of interest.